Digital data transmitter

ABSTRACT

A digital data transmission apparatus includes a transmitting end ( 100 ) that includes: a binary/quadrary conversion unit ( 110 ) for converting a data stream; a coding unit ( 120 ) for mapping converted data to be coded; a digital filter ( 130 ); a D/A conversion unit ( 140 ); a low-pass filter ( 150 ) for attenuating noises which are caused by folding distortion; a differential driver ( 160 ); low-pass filters ( 170   a ) and ( 170   b ) for eliminating noises from differentially outputted signals; and a common mode choke coil ( 180 ) for eliminating common mode noises and outputting a resultant signal to a twisted pair cable ( 300 ), and a receiving end ( 200 ) that includes: a low-pass filter ( 210 ) for eliminating noises from the twisted pair cable; a receiver ( 220 ); an A/D conversion unit ( 230 ); a digital filter ( 240 ); an evaluation unit ( 250 ) for evaluating a signal level of a received signal; a decoding unit ( 260 ) for decoding the signal level into received data; and a synchronization unit ( 270 ) for generating a clock.

TECHNICAL FIELD

The present invention relates to a digital data transmission apparatusand, more particularly, to a digital data transmission apparatus thatreduces emission of noises by a digital filter.

BACKGROUND ART

Some conventional data transmission apparatuses convert digital datainto signal levels of electric signals or optical signals to betransmitted. The transmission rates have been increased through theyears and, recently, some apparatuses transmit large amounts of data,such as video signals, at transmission rates of several tens ofmegabits/sec. The frequencies of these signals are so high that emittednoises cause large problems when these signals are transmitted throughcopper wires or the like.

For example, when such apparatus is mounted on a motor vehicle or thelike, emitted noises may cause malfunctions of other electronicequipment that is mounted on the motor vehicle. Accordingly, there is aneed to make the apparatus hardly emit noises when mounted on thevehicles. It is also required that the apparatus can transmit datacorrectly without being affected by noises emitted from other equipment.Similarly, factory automation machinery or precision machines such asmedical devices also require reduction in noise emission and resistanceto noise.

The conventional data transmission apparatuses utilize a method in whichoptical fiber cables are employed in place of the copper wires so as toemit no electromagnetic waves. When the copper wires are employed, thevoltage of a transmission signal is suppressed at a lower level toreduce emission of noises. There is also employed a method in which atransmission cable for transmitting signals is covered with anothershielded wire to prevent the emitted noises from leaking outside. In thecase of low-speed signal transmission, a transmission cable such as atwisted pair cable that is obtained by twisting two transmission wiresis employed, and signals having opposite polarities are passed throughthe respective wires, so that the signals cancel each other out, wherebynoises are hardly emitted outside. The twisted pair cable has theadvantage in having a simple structure and it can be manufacturedwithout great difficulty and accordingly at a reduced cost, while noiseemission cannot be reduced satisfactorily at high-speed transmission.

In addition, the digital transmission requires communications withhigher reliability. One of factors that reduce the reliability in thedigital transmission is that when the transmission signal constantlytakes the same signal level, synchronization of symbol timing cannot beobtained at the receiving end.

Conventionally, in order to improve the reliability of receipt, thetransmission signal has been processed so that it does not keep ontaking the same level. One of the methods for processing thetransmission signal is scrambling. The scrambling is a method by whichrandom numbers are added to digital data to be transmitted, thereby toprevent the transmission signal from successively taking the same signallevel even when digital data to be transmitted successively take thesame value. In the case of binary transmission in which data aretransmitted by two values, the data are coded according to the bi-phasemark method, thereby to prevent the same signal level from successivelyappearing.

The bi-phase mark coding method is employed as a standard transmissionmethod when digital data of audio data are transmitted. FIG. 13 is adiagram for explaining the bi-phase mark coding method. According to thebi-phase mark coding method, depending on whether the immediatelypreceding symbol is 1 or 0, the next data to be transmitted is codeddifferently, thereby converting 1-bit data to be transmitted into a2-bit symbol. Accordingly, a signal sequence that is coded as shown inFIG. 13 is assured that it never takes the same signal levelsuccessively three or more times. Thus, the symbol timing of thetransmitted data can be detected on the receiving end, whereby the datacan be reproduced correctly.

The data transmission apparatus employing optical fibers emits no noise,while it requires expensive elements such as light-to-electricityconverters or fiber couplers with less optical loss. In addition, theoptical fiber has a problem in its strength, such as limitation in thebend angle of the cable, so that the application range thereof islimited.

Further, according to the method in which the copper signal cable iscovered with a shielded wire, some noises are eliminated by theshielding effect, while the shielded wire between the transmitting andreceiving ends must be grounded sufficiently to provide effectiveshielding, and the prices of connectors, cables, or the like for thatpurpose get higher.

Furthermore, according to the method in which signals having oppositepolarities are passed through a twisted pair cable, when the signals tobe transmitted include higher frequency components, the signals whichflow through two transmission wires of the cable do not always canceleach other out due to slight asymmetry between the two transmissionwires, whereby noises occur unfavorably, so that a sufficient reductionin noises cannot be obtained in the case of high-speed datatransmission.

Thus, the digital signal to be transmitted is conventionally convertedinto a rectangular-wave signal having the corresponding signal level,and then higher frequency components are eliminated by means of alow-pass filter utilizing a resistor, a coil, a capacitor, or the like,thereby to reduce noises. However, it is difficult to give steephigh-band cut-off characteristics to a filter composed of analogelements, without loosing digital information included in signals beingtransmitted, and accordingly the noises cannot be eliminatedsatisfactorily unless the symbol rate of the signal itself issufficiently low.

In utilizing the scrambling in the data transmission apparatus, when adata pattern to be transmitted matches with a random number sequenceemployed at the scrambling, the same signal level would successivelyappear, resulting in that discontinuity of the same signal level cannotalways be assured. While the bi-phase mark method assures thediscontinuity of the same signal level at the binary transmission, whenmulti-valued transmission is performed in cases where several bits ofdata are transmitted at one time, the discontinuity of the same signallevel cannot be obtained. In recent years, demands for multi-valuedtransmission have grown to implement higher-speed digital transmissionor more efficient data transmission in a limited band, and the need fora method for more accurate data transmission at the multi-valuedtransmission has arisen. Further, in order to introduce a newtransmission apparatus, replacement of the conventional transmissionmethod or the like should be taken into consideration. Morespecifically, the new apparatus needs to be able to transmit data of theconventional transmission format without problems and, in the case ofaudio data for example, it is preferable that it can also transmitbi-phase mark data accurately.

Further, like in a case where the data transmission apparatus is mountedon a motor vehicle or the like, when the apparatus is in suchenvironments that the ground levels of connected devices greatly differfrom each other or the fluctuations of the voltage are considerable, itis difficult to correctly transmit the voltage level at the transmittingend to the receiving end. Accordingly, phase modulation or the like isconventionally employed to enable data reproduction even when theabsolute voltage cannot be detected accurately between the transmissionend and the receiving end. However, a modulation method utilizing aspecific carrier frequency unfavorably requires a frequency band that istwice as large as the frequency band of the baseband method that doesnot utilize the modulation.

Besides, in the data communication on motor vehicles, the amount ofelectromagnetic waves emitted from the transmission signal is limited sothat the electromagnetic waves do not cause malfunctions of otherequipment. One of International Standards concerning electromagneticwave noises emitted from the equipment or communication wires on themotor vehicle is CISPR25. CISPR25 defines a limitation value of emittednoises for each frequency and, particularly, there are strictlimitations on signals having frequencies of 30 MHz or higher.Therefore, it is desirable that data should be transmitted in afrequency band of 30 MHz or lower, in which countermeasures against theelectromagnetic waves, such as shielding the signal lines to reducenoises, can be taken without great difficulty. In order to transmit dataefficiently in this frequency band, a data transmission method that isresistant to voltage fluctuations is needed also when the multi-valuedtransmission is performed without using modulation.

The present invention is made to solve the above-mentioned problems, andhas for its object to provide a digital data transmission apparatus thatemits few noises and has a higher resistance to noises, usinginexpensive cables such as twisted pair cable, in data transmission athigh speeds such as above 20 Mbps, and a transmission line coding methodand decoding method in which the same signal level will not successivelyappear also at the multi-valued transmission.

DISCLOSURE OF THE INVENTION

To solve the above-mentioned problems, according to one embodiment ofthe present invention, there is provided a digital data transmissionapparatus comprising: a data coding means for converting digital datainto a signal level corresponding to a symbol that is assigned to thedigital data in each symbol cycle as a prescribed unit cycle; a firstdigital filter that has a first sampling cycle that is shorter than aunit cycle of a signal level string which has been obtained by the datacoding means, and allows only predetermined frequencies to pass; a D/Aconversion means for converting the digital data stream that has passedthrough the digital filter, into an analog signal; a first low-passfilter for eliminating folding distortion of the first digital filterfrom the analog signal obtained by the D/A conversion means, whichdistortion is decided in the first sampling cycle; a differential driverfor converting an output of the first low-pass filter, into two signalshaving opposite polarities relative to a predetermined referencepotential, and differentially outputting the obtained signals; a secondlow-pass filter for eliminating a predetermined frequency band from eachof the signals which are outputted from the differential driver, andoutputting obtained signals to a twisted pair cable; a differentialreceiver for receiving transmission signals transmitted through thetwisted pair cable, and converting a difference in potential between twowires of the cable into a signal; an A/D conversion means for convertingthe signal outputted from the differential receiver into a digitalsignal value in each second sampling cycle; a second digital filter thatallows only a predetermined frequency band of a digital data stream thathas been obtained by sampling of by the A/D conversion means, to pass;and a level evaluation means for evaluating a symbol value from a levelof a signal in symbol timing, including a symbol in the signal, on thebasis of an output from the second digital filter, and converting thesymbol value into corresponding digital data, and the first and seconddigital filters both have low-pass characteristics, and the firstdigital filter has frequency characteristics of cutting off at leastfrequency data which are higher than a frequency band in whichelectromagnetic waves emitted from the respective signals that passthrough the twisted pair cable cancel each other out, thereby toeliminate emission of electromagnetic waves to outside the twisted paircable, and further combination of the first low-pass filter and thesecond low-pass filter has low band cut-off characteristics ofeliminating folding distortion of the first digital filter, which isdecided in the first sampling cycle.

According to another embodiment of the present invention, the digitaldata transmission apparatus discussed above further includes: a commonmode choke coil for eliminating common mode noises from the signals,from which the predetermined frequency band has been eliminated by thesecond low-pass filter, and outputting obtained signals to the twistedpair cable.

According to another embodiment of the present invention, in a digitaldata transmission apparatus, discussed above transmissioncharacteristics of signals which have been passed through the first andsecond digital filters comprises roll-off characteristics.

According to another embodiment of the present invention, in a digitaldata transmission apparatus, discussed above the data coding meansconverts data comprising two or more bits per symbol cycle, into asymbol to be transmitted.

According to another embodiment of the present invention, in a digitaldata transmission apparatus, discussed above the data coding meansincludes signal levels which are more than the number of kinds ofsymbols to be transmitted per symbol cycle, and assigns a symbol in asymbol transmission timing to one of the signal levels.

According, another embodiment of the present invention, in a digitaldata transmission apparatus, discussed above the data coding meansincludes five signal levels, and assigns a symbol in a symboltransmission timing to a signal level other than a previous signal levelcorresponding to a signal which was transmitted in immediately precedingsymbol transmission timing, in the order of 01, 11, 00, 10, startingfrom a lowest signal level.

According to another embodiment of the present invention, in a digitaldata transmission apparatus, discussed above digital data to betransmitted have been coded by a bi-phase mark method, and the datacoding means assigns a symbol in a symbol transmission timing to asignal level other than a previous signal level corresponding to asignal which was transmitted in immediately preceding symboltransmission timing, in the order of 01, 11, 00, 10, starting from alowest signal level, thereby to decide a signal level to be transmitted.

According another embodiment of the present invention, in a digital datatransmission apparatus, discussed above the data coding means includes:a previous signal level storage means for storing a previous signallevel corresponding to the signal which was transmitted in theimmediately preceding symbol transmission timing; and a coding means fordeciding a signal level corresponding to the symbol to be transmitted,on the basis of the previous signal level and the transmission symbol.

According to another embodiment of the present invention, in a digitaldata transmission apparatus, discussed above the coding means assigns asymbol in a symbol transmission timing to a signal level having apredetermined difference from the previous signal level that is storedin the previous signal level storage means.

According to another embodiment of the present invention, in a digitaldata transmission apparatus of, discussed above the data coding means issupplied with a transmission method instruction signal indicatingwhether or not the transmission signal has been coded by the bi-phasemark method.

According to another embodiment of the present invention, in a digitaldata transmission apparatus, discussed above the level evaluation meansincludes: a signal level detection means for detecting a signal level ineach symbol cycle; and a previous signal level storage means for storingthe previous signal level which was received in immediately precedingreceipt timing, and the level evaluation means decodes the signal leveldetected by the signal level detection means, into a correspondingsymbol, on the basis of the previous signal level that is stored in theprevious signal level storage means.

According to another embodiment of the present invention, in a digitaldata transmission apparatus, discussed above the level evaluation meansincludes: a threshold control means for correcting an evaluationthreshold level on the basis of variation values in respective signallevels which were received during a predetermined period; a previoussignal level storage means for storing a previous signal levelcorresponding to a signal which was received in immediately precedingsymbol receipt timing; and a threshold evaluation means for holding athreshold, and performing threshold evaluation for a difference insignal level between a signal level detected in a symbol receipt timingand the previous signal level, thereby to decode a symbol value.

According to another embodiment of the present invention, in a digitaldata transmission apparatus, discussed above the level evaluation meansincludes a synchronization means for establishing synchronization with asymbol cycle of a received signal, and the synchronization meansextracts frequency components having a half cycle as long as the symbolcycle from the received signal, and controls a symbol timing at which asymbol is detected, on the basis of a phase of an extracted signal.

According to another embodiment of the present invention, in a digitaldata transmission apparatus of any, discussed above the level evaluationmeans is supplied with a transmission method instruction signalindicating whether the received signal has been coded by the bi-phasemark method.

According to another embodiment of the present invention, there isprovided a data transmission apparatus including: a data coding meansfor converting digital data into a signal level corresponding to asymbol that is assigned to the digital data in each symbol cycle as aprescribed unit cycle; a first digital filter that has a first samplingcycle that is shorter than a unit cycle of a signal level string whichhas been coded by the data coding means, and allows only predeterminedfrequencies to pass; a D/A conversion means for converting the digitaldata stream that has passed through the digital filter, into an analogsignal; a first low-pass filter for eliminating folding distortion ofthe first digital filter from the analog signal obtained by the D/Aconversion means, which distortion is decided in the first samplingcycle; a differential driver for converting an output of the low-passfilter, into two signals having opposite polarities relative to apredetermined reference potential, and differentially outputting the twosignals; a second low-pass filter for eliminating a predeterminedfrequency band from each of the signals which are outputted from thedifferential driver; and a common mode choke coil for eliminating commonmode noises and outputting obtained signals to a twisted pair cable, andthe first digital filter has frequency characteristics of cutting off atleast frequency data which are higher than a frequency band in whichelectromagnetic waves emitted from the respective signals that passthrough the twisted pair cable cancel each other out, thereby toeliminate emission of the electromagnetic waves to outside the twistedpair cable, and further combination of the first low-pass filter and thesecond low-pass filter has low band cut-off characteristics ofeliminating folding distortion of the first digital filter, which isdecided in the first sampling cycle.

Thus, according to another embodiment of the present invention, whereinthere is provided a digital data transmission apparatus comprising: adata coding means for converting digital data into a signal levelcorresponding to a symbol that is assigned to the digital data in eachsymbol cycle as a prescribed unit cycle; a first digital filter that hasa first sampling cycle that is shorter than a unit cycle of a signallevel string which has been obtained by the data coding means, andallows only predetermined frequencies to pass; a D/A conversion meansfor converting the digital data stream that has passed through thedigital filter, into an analog signal; a first low-pass filter foreliminating folding distortion of the first digital filter from theanalog signal obtained by the D/A conversion means, which distortion isdecided in the first sampling cycle; a differential driver forconverting an output of the first low-pass filter, into two signalshaving opposite polarities relative to a predetermined referencepotential, and differentially outputting the obtained signals; a secondlow-pass filter for eliminating a predetermined frequency band from eachof the signals which are outputted from the differential driver, andoutputting obtained signals to a twisted pair cable; a differentialreceiver for receiving transmission signals transmitted through thetwisted pair cable, and converting a difference in potential between twowires of the cable into a signal; an A/D conversion means for convertingthe signal outputted from the differential receiver into a digitalsignal value in each second sampling cycle; a second digital filter thatallows only a predetermined frequency band of a digital data stream thathas been obtained by sampling of by the A/D conversion means, to pass;and a level evaluation means for evaluating a symbol value from a levelof a signal in symbol timing, including a symbol in the signal, on thebasis of an output from the second digital filter, and converting thesymbol value into corresponding digital data, and the first and seconddigital filters both have low-pass characteristics, and the firstdigital filter has frequency characteristics of cutting off at leastfrequency data which are higher than a frequency band in whichelectromagnetic waves emitted from the respective signals that passthrough the twisted pair cable cancel each other out, thereby toeliminate emission of electromagnetic waves to outside the twisted paircable, and further combination of the first low-pass filter and thesecond low-pass filter has low band cut-off characteristics ofeliminating folding distortion of the first digital filter, which isdecided in the first sampling cycle, therefore a higher transmissionrate can be realized. In addition, a frequency band of the transmissionsignal can be limited to a frequency band having a noise eliminationeffect that is achieved when the signals having opposite polarities arepassed through the twisted pair cable, thereby removing almost allelectromagnetic noises also at high-speed data transmission. Further,noise elimination characteristics required for a low-pass filter can bedividedly given to two low-pass filters, i.e., the first low-pass filterand the second low-pass filter, so that there is no need to give steepattenuation characteristics to the respective low-pass filters,resulting in uncomplicated constructions and reduced product costs ofthe filters. Furthermore, noises which are caused by the distortion ofthe differential driver can be eliminated by the second low-pass filter.

According to another embodiment of the present invention, wherein adigital data transmission apparatus discussed above further includes: acommon mode choke coil for eliminating common mode noises from thesignals, from which the predetermined frequency band has been eliminatedby the second low-pass filter, and outputting obtained signals to thetwisted pair cable, common mode noises which occur in both wires of thetwisted pair cable can be eliminated.

According to another embodiment of the present invention, wherein in adigital data transmission apparatus, discussed above transmissioncharacteristics of signals which have been passed through the first andsecond digital filters comprises roll-off characteristics, the signalwhich has passed through the first digital filter and the second digitalfilter can be converted into a signal within a band that is slightlylarger than half of the symbol rate. In addition, the signal isconverted into a signal having no interference between adjacent codes insymbol timing, whereby a symbol included in the signal can be read insymbol timing.

According to another embodiment of the present invention, wherein in adigital data transmission apparatus, discussed above the data codingmeans converts data comprising two or more bits per symbol cycle, into asymbol to be transmitted, the symbol rate can be lowered, therebyrealizing a high transmission rate. Further, each time one symbol iscoded, a signal level that represents the symbol can be transmitted,thereby realizing data transmission with little delay.

According to of the present invention, wherein in a digital datatransmission apparatus, discussed above the data coding means includessignal levels which are more than the number of kinds of symbols to betransmitted per symbol cycle, and assigns a symbol in a symboltransmission timing to one of the signal levels, the symbol can beassigned to a predetermined signal level to be coded. Further, thesymbol can be converted into a signal level that is different from theprevious signal level, whereby the signals outputted from thetransmitting end constantly vary with symbol timing, and synchronizationcan be readily obtained on the receiving end.

According to another embodiment of the present invention, wherein in adigital data transmission apparatus, discussed above the data codingmeans includes five signal levels, and assigns a symbol in a symboltransmission timing to a signal level other than a previous signal levelcorresponding to a signal which was transmitted in immediately precedingsymbol transmission timing, in the order of 01, 11, 00, 10, startingfrom a lowest signal level, the symbol can be assigned to apredetermined signal level to be coded.

According to another embodiment of the present invention, wherein in adigital data transmission apparatus, discussed above digital data to betransmitted have been coded by a bi-phase mark method, and the datacoding means assigns a symbol in a symbol transmission timing to asignal level other than a previous signal level corresponding to asignal which was transmitted in immediately preceding symboltransmission timing, in the order of 01, 11, 00, 10, starting from alowest signal level, thereby to decide a signal level to be transmitted,the data coded by the bi-phase mark method can be decoded to obtain asymbol by evaluating a signal only by an evaluation as to whether thedata is higher or lower than one threshold, like in the binarytransmission, whereby signal detection having a reliability that isquite close to the binary evaluation is performed. Further, possiblevalues taken in the respective symbol timing are binary, and thedistance between symbols is 2 or more symbols across the signal level 2,so that the possibility of error evaluation caused by noises can bereduced to a level as low as the binary transmission.

According to another embodiment of the present invention, wherein in adigital data transmission apparatus of, discussed above the data codingmeans includes: a previous signal level storage means for storing aprevious signal level corresponding to the signal which was transmittedin the immediately preceding symbol transmission timing; and a codingmeans for deciding a signal level corresponding to the symbol to betransmitted, on the basis of the previous signal level and thetransmission symbol, a symbol can be assigned to a predetermined signallevel to be coded. In addition, the symbol can be converted into asignal level that is different from the previous signal level.

According to another embodiment of the present invention, wherein in adigital data transmission apparatus, discussed above the coding meansassigns a symbol in a symbol transmission timing to a signal levelhaving a predetermined difference from the previous signal level that isstored in the previous signal level storage means, the receiving end canevaluate a symbol of a received signal only on the basis of a differencein signal level from the previous signal level. Further, datatransmission can be performed correctly also in cases where thetransmitting end and the receiving end have different voltage levels atmulti-valued transmission of the baseband, or in environments wherefluctuations in the voltage are large.

According to another embodiment of the present invention, wherein in adigital data transmission apparatus of, discussed above the data codingmeans is supplied with a transmission method instruction signalindicating whether or not the transmission signal has been coded by thebi-phase mark method, the transmission signal can be coded in accordancewith the transmission method, whereby data based on the bi-phase markmethod as a conventional transmission method can be transmitted.

According to another embodiment of the present invention, wherein in adigital data transmission apparatus, discussed above the levelevaluation means includes: a signal level detection means for detectinga signal level in each symbol cycle; and a previous signal level storagemeans for storing the previous signal level which was received inimmediately preceding receipt timing, and the level evaluation meansdecodes the signal level detected by the signal level detection means,into a corresponding symbol, on the basis of the previous signal levelthat is stored in the previous signal level storage means, the symbol ofthe received signal can be evaluated on the basis of the previous signallevel and the signal level of the received signal. Further, each timeone signal level is received, a symbol represented by the signal levelcan be obtained, thereby realizing data receipt with little delay.

According to another embodiment of the present invention, wherein in adigital data transmission apparatus, discussed above the levelevaluation means includes: a threshold control means for correcting anevaluation threshold level on the basis of variation values inrespective signal levels which were received during a predeterminedperiod; a previous signal level storage means for storing a previoussignal level corresponding to a signal which was received in immediatelypreceding symbol receipt timing; and a threshold evaluation means forholding a threshold, and performing threshold evaluation for adifference in signal level between a signal level detected in a symbolreceipt timing and the previous signal level, thereby to decode a symbolvalue, a symbol of a received signal can be evaluated only on the basisof the difference in signal level from the previous signal level. Thus,for example when the transmitting end and the receiving end havedifferent potentials or when the potentials vary, the data can becorrectly decoded even when the absolute voltage level on thetransmitting end cannot be detected. Further, the threshold is modifiedon the basis of the evaluation results on signals which have beenreceived for a predetermined time period, so that correct data can beobtained by modifying the threshold in cases where transmitted voltagesvary according to fluctuations in the supply voltage, or the like.

According to another embodiment of the present invention, wherein in adigital data transmission apparatus, discussed above the levelevaluation means includes a synchronization means for establishingsynchronization with a symbol cycle of a received signal, and thesynchronization means extracts frequency components having a half cycleas long as the symbol cycle from the received signal, and controls asymbol timing at which a symbol is detected, on the basis of a phase ofan extracted signal, a more reliable synchronization can be obtainedutilizing changes in the signal level of the received signal.

According to another embodiment of the present invention, wherein in athe digital data transmission apparatus of, discussed above the levelevaluation means is supplied with a transmission method instructionsignal indicating whether or not the received signal has been coded bythe bi-phase method, the transmitted data can be decoded in accordancewith the transmission method, whereby data based on the bi-phase markmethod as a conventional transmission method can be received.

According to another embodiment of the present invention, wherein thereis provided a data transmission apparatus including: a data coding meansfor converting digital data into a signal level corresponding to asymbol that is assigned to the digital data in each symbol cycle as aprescribed unit cycle; a first digital filter that has a first samplingcycle that is shorter than a unit cycle of a signal level string whichhas been coded by the data coding means, and allows only predeterminedfrequencies to pass; a D/A conversion means for converting the digitaldata stream that has passed through the digital filter, into an analogsignal; a first low-pass filter for eliminating folding distortion ofthe first digital filter from the analog signal obtained by the D/Aconversion means, which distortion is decided in the first samplingcycle; a differential driver for converting an output of the low-passfilter, into two signals having opposite polarities relative to apredetermined reference potential, and differentially outputting the twosignals; a second low-pass filter for eliminating a predeterminedfrequency band from each of the signals which are outputted from thedifferential driver; and a common mode choke coil for eliminating commonmode noises and outputting obtained signals to a twisted pair cable, andthe first digital filter has frequency characteristics of cutting off atleast frequency data which are higher than a frequency band in whichelectromagnetic waves emitted from the respective signals that passthrough the twisted pair cable cancel each other out, thereby toeliminate emission of the electromagnetic waves to outside the twistedpair cable, and further combination of the first low-pass filter and thesecond low-pass filter has low band cut-off characteristics ofeliminating folding distortion of the first digital filter, which isdecided in the first sampling cycle, a high transmission rate can beobtained. In addition, the frequency band of the transmission signal canbe limited to a frequency band having a noise elimination effect that isachieved when the signals having opposite polarities are passed througha twisted pair cable, thereby removing almost all electromagnetic noisesalso at the high-speed transmission. Further, there is no need to givesteep attenuation characteristics to the first low-pass filter and thesecond low-pass filter, resulting in uncomplicated constructions andreduced product costs of the filters. Further, noises caused by thedistortion of the differential driver can be eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a structure of a digital datatransmission apparatus according to a first embodiment of the presentinvention.

FIG. 2 is a block diagram illustrating a structure of a coding unitaccording to the first embodiment.

FIG. 3 is a diagram for explaining a coding process by a signalconversion unit according to the first embodiment.

FIG. 4 is a diagram for explaining an evaluation process by anevaluation unit according to the first embodiment.

FIG. 5 is a block diagram illustrating a structure of a decoding unitaccording to the first embodiment.

FIG. 6 is a diagram for explaining a decoding process by a signalconversion unit according to the first embodiment.

FIG. 7 is a block diagram illustrating a structure of a synchronizationunit according to the first embodiment.

FIG. 8 is a diagram for explaining a noise elimination effect accordingto the first embodiment.

FIG. 9 is a diagram for explaining possible values in a case where dataare coded by a bi-phase mark method according to the first embodiment.

FIG. 10 is a diagram for explaining symbol arrangement on the basis of adifference from the previous value, as another example of the codingprocess by the signal conversion unit according to the first embodiment.

FIG. 11 is a diagram for explaining another example of the evaluationprocess by the evaluation unit according to the first embodiment.

FIG. 12 is a block diagram for explaining another structure of theevaluation unit according to the first embodiment.

FIG. 13 is a diagram for explaining a coding method according to abi-phase mark method as a conventional transmission method.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be describedwith reference to the drawings. The embodiment shown here is onlyexemplary, and the invention is not limited to this embodiment.

[Embodiment 1]

Initially, a digital data transmission apparatus and a data transmissionapparatus will be described as a first embodiment, with reference to thedrawings.

FIG. 1 is a block diagram illustrating a structure of a digital datatransmission apparatus according to the first embodiment.

As shown in FIG. 1, the digital data transmission apparatus according tothe first embodiment includes a transmitting end 100 for transmittingdata, and a receiving end 200 for receiving the data transmitted fromthe transmitting end 100, which are connected with each other through atwisted pair cable 300.

The transmitting end 100 includes a binary-to-quadrary conversion unit110 for converting a 1-bit data stream into a 2-bit (four-valued) datastream; a coding unit 120 for mapping 2-bit data that is obtained by thebinary/quadrary conversion unit to a predetermined signal level to becoded; a digital filter 130 that allows a band of frequency componentscorresponding to half of a symbol rate to pass; a D/A conversion unit140 for converting the 2-bit data that has passed through the digitalfilter 130 into an analog signal; a low-pass filter 150 for attenuatingnoises due to folding distortion of the digital filter; a differentialdriver 160 for converting the analog signal that has passed through thelow-pass filter 150 into two signals having opposite polarities relativeto a reference potential, and differentially outputting the two signalstoward the twisted pair cable 300; low-pass filters 170 a and 170 b thateliminate noises from the respective signals which are outputted fromthe differential driver 160; and a common mode choke coil 180 thateliminates common mode noises that occur in the respective signal lines.

The receiving end 200 includes a low-pass filter 210 that eliminatesnoises outside the signal band of a transmission signal for both wiresof the twisted pair cable 300; a differential receiver 220 for receivinga signal that has passed through the low-pass filter 210; an A/Dconversion unit 230 for converting the received signal into a digitalsignal; a digital filter 240 that allows only a predetermined frequencyband to pass; an evaluation unit 250 for evaluating the level of thereceived signal; a decoding unit 260 for decoding the signal levelevaluated by the evaluation unit 250 into 2-bit receipt data; and asynchronization unit 270 for generating a clock that is employed at theA/D conversion.

The operation of the digital data transmission apparatus that isconstructed as described above will be described.

A digital signal that is transmitted through the transmitting end 100 isinitially inputted to the binary/quadrary conversion unit 110. Thebinary/quadrary conversion unit 110 converts a 1-bit data stream into a2-bit (four-valued) data stream, i.e., “01”, “11”, “00”, or “10”, andtransmits the 2-bit data stream to the coding unit 120.

The coding unit 120 performs a coding process by mapping the signalinputted from the binary/quadrary conversion unit 110 to a signal levelthat represents the value of the signal. This coding unit 120 includes,as shown in FIG. 2, a previous value storage unit 121 for storing aprevious value that is obtained by coding the immediately precedingdata, and a signal conversion unit 122 for performing a coding processon the basis of the previous value that is stored in the previous valuestorage unit 121 and the signal outputted from the binary/quadraryconversion unit 110. The signal conversion unit 122 maps the signal to asignal level other than the level of the signal that has been codedimmediately before, with reference to a conversion table as shown inFIG. 3. The conversion table shown in FIG. 3 defines the signal levelsto which the symbols to be transmitted, i.e., “01”, “11”, “00”, or “10”,are mapped on the basis of the previous signal level (0 to 4), so thatthe symbols are mapped to signal levels that are different from theprevious levels. For example, when the previous value stored in theprevious value storage unit 121 is a signal level “0”, and a symbol “01”is newly inputted from the binary/quadrary conversion unit 110, thesignal conversion unit 122 converts this symbol into a signal level “1”.The inputted signals are similarly mapped to four-value signal levelsother than the respective previous signal levels. Thus, the coding unit120 encodes the signal into a signal level that is different from theprevious signal level in any case.

Frequency components of the coded signal, which are higher than half ofthe symbol rate, are eliminated by the digital filter 130. This digitalfilter 130 is a low-pass filter that allows a band of frequencycomponents corresponding to half of the symbol rate to pass, and thisdigital filter is practically constructed so as to provide appropriateroll-off characteristics in conjunction with the digital filter 240 onthe receiving end 200. The transmission of pulse signals requires aninfinite bandwidth, but when the signal is passed through a filterhaving the roll-off characteristics, the signal is turned into a signalwithin a band that is slightly larger than half of the symbol rate, andconverted into a signal without interference between adjacent codes inthe reading timing. Accordingly, the data to be transmitted is convertedinto a signal in a limited band.

The signal that has passed through the digital filter 130 is convertedinto an analog signal by the D/A conversion unit 140. That is, thesignal is converted into a signal that includes a coded symbol in symboltiming of each symbol cycle. Folding frequency components of the analogsignal, which appear in a band of frequencies that are twice as high asthe frequencies that have passed through the digital filter 130 orhigher frequencies, are attenuated by the low-pass filter 150, and theobtained signal is transmitted to the differential driver 160. Thedifferential driver 160 converts the inputted signal into two signalshaving amplitudes which are proportional to the inputted signal andopposite polarities relative to a reference potential, and transmits theobtained two signals to the low-pass filters 170 a and 170 b,respectively.

The low-pass filters 170 a and 170 b have the same attenuationcharacteristics. Further, combination of the low-pass filter 150 and thelow-pass filter 170 a or 170 b has low-band cutoff characteristics foreliminating noises which are caused by folding distortion of the digitalfilter 130. It has certain pass amplitude characteristics and delaycharacteristics in the transmission signal band, and realizes sufficientcutoff characteristics in a band in which primary folding distortioncomponents of the digital filter 130 will appear. For example, when thesampling cycle of the digital filter 130 is four times as high as thesymbol rate, the approximately 100 dB attenuation characteristics arerequired in a band that is six times as high as the symbol rate, inwhich the primary folding distortion components will appear. In thiscase, 100 dB attenuation characteristics are dividedly given to thelow-pass filter 150 and the low-pass filter 170 a or 170 b.

The low-pass filters 170 a and 170 b eliminate the folding distortionsof the digital filter 130 and noises caused by the differential driver160, from the inputted signal. Then, the common mode choke coil 180eliminates common mode noises of the signal, and outputs the obtainedsignal to the twisted pair cable 300.

The signal that is transmitted through the twisted pair cable 300 is asignal that is always coded so as to have a signal level different fromthe previous signal level, and this signal has values which constantlyvary with symbol timing.

Then, the receiving end 200 makes the transmission signals which havebeen transmitted through both wires of the twisted pair cable 300 passthrough the low-pass filter 210, to eliminate noises outside apredetermined signal band. For example, larger noises in the range ofsome kilohertz to one GHz may be contained when the apparatus is mountedon a motor vehicle and, when such high-frequency noises are contained,the frequency characteristics of the differential receiver 220 in thenext stage cannot compensate for desired characteristics. Accordingly,the low-pass filter 210 cuts off components of an area in which thefrequency characteristics of the differential receiver 220 cannotcompensate for the desired characteristics, as well as converts thesignal into a signal of a band which can be processed by the digitalfilter 240 that is connected in the later stage.

Then, the signal from which the noises have been eliminated by thelow-pass filter 210 is received by the differential receiver 220. Thedifferential receiver 220 outputs a signal which is proportional to adifference signal between the both wires of the twisted pair cable 300,and then the outputted signal is converted into a digital signal by theA/D conversion unit 230. Here, the synchronization unit 270 generates asynchronized sampling clock and transmits the sampling clock to the A/Dconversion unit 230, so that the A/D conversion unit 230 performssampling in the symbol timing.

The synchronization unit 270 includes a band pass filter 271, a D/Aconverter 272, a comparator 273, a PLL 274, and a frequency divider 275,as shown in an example of the construction of FIG. 7. Here, thefrequency division rate of the frequency divider 275 is decideddepending on how many times the sampling cycle is as high as the symbolcycle. For example, when the sampling rate is twice as high as thesymbol rate, the frequency is divided by four. The synchronization unit270 makes clock synchronization utilizing the signal levels of thereceived signal, which constantly vary with the symbol cycle. Frequencycomponents corresponding to half of the symbol rate are extracted fromthe received signal by the band pass filter 271, then the signal isconverted into an analog signal by the D/A converter 272, and the analogsignal is converted into a square wave signal by the comparator 273.This square wave signal is inputted to the PLL 274 as a reference clock(REF), and phase comparison is performed between the reference clock anda clock (VAL) that is obtained by dividing the frequency of a clockoutputted from the PLL 274 in the frequency divider 275, therebyestablishing the clock synchronization. Consequently, the A/D conversionunit 230 samples the signal in the symbol timing, thereby to convertsthe analog signal into a digital signal. The structure of thesynchronization unit 270 is not limited to that shown in FIG. 7, andreproduction synchronization can be easily realized on the receiving endutilizing the fact that the values of other means also constantly varywith the symbol cycle.

The obtained digital signal is passed through the digital filter 240.The digital filter 240 provides roll off characteristics in conjunctionwith the digital filter 130 on the transmitting end 100, and convertsthe passed digital signal into a signal that has no interference betweenadjacent codes and can be read in appropriate timing.

Then, the evaluation unit 250 evaluates the level of the signal that hasbeen sampled in symbol timing, to decide a signal level from five-valuelevels. This evaluation process is carried out as shown in FIG. 4, andthe evaluation unit 250 evaluates the level of the sampled signal todecide one of signal levels 0, 1, 2, 3 and 4, on the basis of thresholds1, 2, 3, and 4.

The decoding unit 260 converts the signal level that has been evaluatedby the evaluation unit 250 into 2-bit receipt data. This decoding unit260 includes, as shown in FIG. 5, a previous value storage unit 261 forstoring a previous signal level that has been evaluated by theevaluation unit 250 in the immediately preceding symbol timing, and asignal conversion unit 262 for performing a decoding process on thebasis of the signal level stored in the previous value storage unit 261and the signal level outputted from the evaluation unit 250. The signalconversion unit 262 decodes the signal level into a symbol withreference to the conversion table shown in FIG. 6. The conversion tablein FIG. 6 is identical to the table that is employed at the coding bythe coding unit 120 of the digital data transmission apparatus, and thereceipt data is obtained with reference to the same conversion table asemployed at the conversion in the digital data transmission apparatus.For example, when the previous value stored in the previous valuestorage unit 261 is the signal level 0 and when a signal level 4 (setvalue) is newly inputted from the evaluation unit 250, the signalconversion unit 262 converts the signal level into a symbol “10”.

Now, a description will be given of a large reduction in electromagneticwaves emitted from the twisted pair cable 300 as the transmission cableof the digital data transmission apparatus according to the firstembodiment.

One of the International Standards associated with electromagnetic wavenoises emitted from equipment or communication lines on motor vehiclesis CISPR25. CISPR25 defines a limitation value on the emitted noises foreach frequency.

For example, in the case of a balanced transmission through a twistedpair cable without shielding, a limitation value is defined for afrequency band of 30 MHz or higher, in which the amount of emittednoises is relatively difficult to reduce. Thus, when signals in afrequency band of 30 MHz or higher are transmitted, it is difficult tosatisfy the on-vehicle requirements. A limitation value for the emittednoises is defined also for a band of 30 MHz or lower, while in this caseit is possible to reduce the amount of noises by keeping balancing.Therefore, when the frequency band of the transmission signal is reducedto 30 MHz or lower, the amount of emitted noises which complies with theon-vehicle requirements can be obtained.

The twisted pair cable 300 has errors in the twisting pitch or thelength of a wire connecting to the driver, and accordingly thetransmission signal is slightly out of phase. The influence of the phaseshifting gets larger as the frequencies of the signals to be transmittedare higher, and the signals do not cancel out the emitted noises. Thus,the digital filter performs a band limitation so that the signal bandsof transmission signals are within the range of frequencies in which theemitted noises sufficiently cancel each other out.

FIG. 8 is a diagram for explaining the relation between the noiseelimination effect and the frequency level at the transmission through atwisted pair cable. The noise elimination effects of the twisted paircable vary with the production precision, but the noise eliminationeffect is noticeably reduced when the frequency is beyond approximately30 MHz. Accordingly, the digital filter limits the signal band to below30 MHz.

The digital filter 130 samples the signal at a frequency higher than thesymbol rate. Then, the digital filter 130 sets frequency characteristicsso that the combined characteristics of the digital filter 130 and thedigital filter 240 have roll-off characteristics relative to half of thesymbol rate, and the obtained characteristics are equally divided andgiven to the digital filters 130 and 240. A digital filter having suchcharacteristics may be constituted by tens of FIR (FINITE IMPULSERESPONSE) digital filters having coefficients.

The digital filters 130 and 240 are low-pass filters that allowfrequencies up to a level slightly higher than half of the symbol rate,to pass.

Therefore, signals having opposite polarities of a frequency band thatis slightly higher than half of the symbol rate (which are accuratelycalculated on the basis of the rate of the roll-off characteristics,i.e., the percentage) are transmitted through the twisted pair cable300. Then, the signals having opposite polarities are passed through twotransmission wires of the twisted pair cable 300, then cancel theelectromagnetic waves emitted therefrom each other out, resulting inalmost no noise emission.

When 48 Mbps transmission is to be performed, the number of bitstransmitted per symbol is set at 2 bits, and the symbol rate in thiscase is 24 MHz.

Further, when the roll off filter that is constituted by the digitalfilters 130 and 240 includes the roll off characteristics ofapproximately 15% relative to 12 MHz, the signal band can be limited toapproximately 15 MHz. Such characteristics are realized by utilizingcharacteristics of a digital filter that enables to flexibly designfrequency characteristics or phase characteristics and can realize steepfrequency characteristics with ideal phase characteristics, thereby tolimit a band of the data to a frequency band of the twisted pair cablehaving the noise elimination effect. Further, the multi-valuedtransmission enables to lower the symbol rate, thereby realizing ahigher transmission rate.

As described above, according to the digital data transmission apparatusand the transmission line coding method and decoding method of the firstembodiment, signal levels which are more than the number of symbols tobe transmitted are provided, and the signal level representing eachsymbol in each symbol transmission timing is mapped to a signal levelother than the signal level that was transmitted in the previous symboltiming. Thus, in any case, the signal can be converted into a signallevel that is different from the previous signal level and, accordingly,the signals outputted from the transmitting end 100 always have valuesvarying with the symbol cycle, thereby facilitating the synchronizationon the receiving end 200. Further, the signal level that represents asymbol is transmitted each time the symbol is coded, thereby enablingthe data transmission with little delay.

In addition, the digital filter 130 and the digital filter 240constitute a filter having appropriate roll off characteristics, wherebythe signal that has passed through the filters is converted into asignal within a frequency band that is slightly larger than half of thesymbol rate. Further, the signal is converted into a signal withoutinterference between adjacent codes in predetermined timing, so that acode included in the signal can be read in predetermined timing.

The coding unit 120 converts a signal into a symbol that enablestransmission of data comprising 2 or more bits per symbol timing,whereby efficient data transmission can be performed in a limitedfrequency band.

In the coding unit 120, the previous signal level is stored in theprevious value storage unit 121, and the signal conversion unit 122encodes a symbol to be transmitted on the basis of the previous signallevel. Therefore, the signal can be mapped to a signal level other thanthe signal level which was transmitted in the previous symbol timing,whereby the signal is converted into a signal level that is differentfrom the previous signal level in any case.

In the decoding unit 260, the previous signal level is stored in theprevious value storage unit 261, and the signal conversion unit 262decodes the received signal level on the basis of the previous signallevel, whereby the transmitted symbol is obtained from the receivedsignal level. In addition, a symbol that is represented by a signallevel is obtained each time the signal level is received, therebyenabling the data receipt with little delay.

Further, 2-bit data is transmitted per symbol and the number of symbolsto be transmitted is set at four, so that efficient data transmission isperformed in a limited band.

Further, signal levels which are more than the number of symbols to betransmitted are provided, so that the previous signal level isprohibited and the symbol to be transmitted is mapped to the signallevel other than the previous value, whereby continuous outputs of thesame signal level is avoided.

Furthermore, the symbols to be transmitted are mapped to the signallevels in the order of “01”, “11”, “00”, and “10” starting from thelowest signal level, so that the symbols can be mapped to thecorresponding predetermined signal levels.

The symbols are made correspond to the respective signal levels otherthan the previous signal level on the basis of the signal level of theimmediately preceding detected signal, to decode the detected signallevel into a symbol, whereby the transmitted symbol can be obtained fromthe received signal level. Further, a symbol that is represented by asignal level can be obtained each time the signal level is received,thereby realizing the data receipt with little delay.

The synchronization unit 270 extracts, from a receipt signal, frequencycomponents having a cycle that is half of the symbol cycle signal, andcontrols the symbol extraction timing on the basis of the phase of theextracted signal, whereby a more reliable synchronization can heestablished utilizing changes in the signal level of the receipt signal.

Further, the noise elimination characteristics that are needed by thelow-pass filter are dividedly given to the two low-pass filters, i.e.,the low-pass filter 150 and the low-pass filter 170 a or 170 b, so thatthere is no need to give steep attenuation characteristics to each ofthe low-pass filters, resulting in a uncomplicated structure of thefilter, and a reduced production cost. In addition, noises caused by thedistortion of the differential driver 160 can be eliminated.

In the digital transmission of audio data, the data that has been codedaccording to the bi-phase mark method are transmitted using a plasticoptical fiber or the like. It is envisioned that these signals that havebeen coded by the bi-phase mark method are transmitted/received also bythe digital data transmission apparatus of the present invention. FIG. 9is a diagram showing possible values in the case where the signals arecoded by the bi-phase mark method. In this figure, symbols other thancircled and boxed symbols will not be coded. Values which can be takenin each symbol timing are binary, and the distance between adjacentsymbols is 2 or more symbols across the signal level 2.

When the digital data transmission apparatus according to this inventiontransmits/receives data corresponding a bit string or data that has beencoded by the bi-phase mark method, a transmission method instructionsignal is inputted to the coding unit 120 and the evaluation unit 250,to switch between a case of transmitting simply a bit string and a caseof transmitting the data that has been coded by the bi-phase markmethod.

When the transmission method instruction signal indicates the bi-phasemark method, the coding unit 120 encodes an inputted symbol withreferring to the conversion table shown in FIG. 9.

When the transmission method instruction signal indicates the bi-phasemark method, the evaluation unit 250 decides whether a received signalis higher or lower than threshold 5 shown in FIG. 4. When the previoussignal level is 0, and when the received signal is higher than thethreshold 5 the signal level of the received signal is set at 3, whilethe signal level is set at 1 when the received signal is lower than thethreshold 5. Similarly, when the previous signal level is 1 and when thereceived signal is higher than the threshold 5 the signal level of thereceived signal is set at 3, while the signal level is set at 0 when thereceived signal is lower than the threshold 5. When the previous signallevel is 3, and when the received signal is higher than the threshold 5the signal level of the received signal is set at 4, while the signallevel is set at 1 when the received signal is lower than the threshold5. When the previous signal level is 4, and when the received signal ishigher than the threshold 5 the signal level of the received signal isset at 3, while the signal level is set at 1 when the received signal islower than the threshold 5.

Thus, the transmission/receipt of data which have been coded accordingto the bi-phase mark method can realize resistance to noises that isquite close to the binary transmission. On the receiving end 200, athreshold at the threshold evaluation with the previous signal level andthe signal level of the received signal is set at threshold 5, and thesymbol can be decoded by evaluating the signal only by deciding whetherit is higher or lower than a threshold, like in the binary transmission,thereby realizing signal detection having a reliability that is quiteclose to the binary evaluation. Further, possible values taken in eachsymbol timing are binary, and the distance between symbols is two ormore symbols across the signal level 2, so that the possibility oferrors caused by noises can be suppressed to a level as low as thebinary transmission.

In this embodiment, the four-value symbol is converted into five-valuesignal levels, while also in cases where the four-value symbol isconverted into multivalue signal levels which are more than four, suchas 8-value or 16-value, the similar method is used to prevent a codefrom being mapped to the signal level of the immediately precedingsignal, and accordingly the same effects are obtained.

In this embodiment, the coding by the coding unit 120 is performed withreference to the conversion table as shown in FIG. 3, while the codingis not restricted to this method, but the coding can be performed withreference to a conversion table as shown in FIG. 10.

The conversion table shown in FIG. 10 defines signal levels to whichsymbols to be transmitted are mapped, on the basis of a difference insignal level from the previous signal level corresponding to theimmediately preceding transmitted signal. More specifically, when asymbol “10” is transmitted, this symbol is mapped to a signal level thatis larger than the previous signal level by one level, or a signal levelsmaller than the previous signal level by four levels. Similarly, asymbol “00” is mapped to a signal level that is larger than the previoussignal level by two levels, or a signal level smaller than the previoussignal level by three levels. A symbol “11” is mapped to a signal levelthat is larger than the previous signal level by three levels, or asignal level smaller than the previous signal level by two levels. Asymbol “01” is mapped to a signal level that is larger than the previoussignal level by four levels, or a signal level smaller than the previoussignal level by one level.

When the signal level that has been coded on the basis of the differencein signal level is received to be decoded, the evaluation unit 250detects a difference from the previous signal level corresponding to theimmediately preceding received signal, thereby to obtain a symbol. Morespecifically, the previous signal level is stored, then a differencebetween the stored signal level and the received signal level isobtained, and the obtained difference is evaluated on the basis ofthresholds 1 to 7 as shown in FIG. 11, thereby to assigning one ofsignal evaluation values “−4” to “+4” to the received signal. Then, theobtained signal evaluation value is transmitted to the decoding unit260. The decoding unit 260 decodes the signal evaluation values “−4”,“−3”, “−2”, “−1”, “+1”, “+2”, “+3”, and “+4” into symbols “01”, “11”,“00”, “10”, “01”, “11”, “00”, and “10”, respectively.

Thus, the symbol of the transmitted signal can be decided only on thebasis of the difference in signal level from the previous signal, sothat for example when the transmitting end 100 and the receiving end 200have different potentials or the potentials vary, and even when theabsolute voltage level cannot be detected on the transmitting end 100,the data can be correctly decoded by detecting the difference from theimmediately preceding received signal level. Further, the transmittingend 100 always maps a symbol to a signal level other than the previoussignal level and transmits the obtained symbol, so that the voltagesconstantly vary with symbol. Therefore, it is satisfactory that thereceiving end 200 detects alternating components (the voltagefluctuation level), and when the potential difference between thetransmitting end 100 and the receiving end 200 is quite large, a circuitfor cutting off the alternating components can be provided in thereceiving end. This is useful in circumstances where the transmittingend and the receiving end have different ground levels or where thevoltage-resistant characteristics are demanded, such as in a case wherethe apparatus is mounted on a motor vehicle.

At the coding on the basis of the signal level difference, data whichhas been coded by the bi-phase mark method may be mapped to a signallevel with referring to the conversion table shown in FIG. 10. When thedata that has been coded by the bi-phase mark method is coded on thebasis of the difference from the previous signal level, the distancebetween symbols is always two ore more levels, so that the evaluation isperformed by setting a threshold at an intermediate signal level betweenrespective possible symbols, thereby to realize the data receipt withhigher precision.

Further, other than the coding of symbols with reference to theconversion table as shown in FIG. 10, another conversion table may beemployed so long as symbols are mapped on the basis of a differencebetween the previous signal level and the next assumable signal level.

When a signal level that has been coded on the basis of the signal leveldifference is received and decoded, the evaluation unit 250 may beconstructed as shown in FIG. 12. FIG. 12 is a block diagram illustratinganother construction of the evaluation unit 250. This evaluation unit250 includes a threshold evaluation unit 251 that holds a threshold andsubjects a signal that has passed through the digital filter 240 tothreshold evaluation, a threshold control unit 252 that controls thethreshold, and a previous value storage unit 253 that stores theimmediately preceding signal level.

The threshold evaluation unit 251 calculates a difference between thesignal level of a signal that has passed through the digital filter 240and the previous signal level that is stored in the previous valuestorage unit 253, and evaluates the obtained difference on the basis ofthresholds 1 to 7 as shown in FIG. 11, thereby assigning one of signalevaluation values “−4” to “+4” to the signal. Then, the variation in thesignal level (signal evaluation value) is transmitted to the decodingunit 260, as well as the present signal level is stored in the previousvalue storage unit 253, and the difference between the signal evaluationvalue (evaluation result) and the previous signal level is transmittedto the threshold control unit 252. Then, the threshold control unit 252calculates differences corresponding to one variation in the threshold,on the basis of the received difference and the signal evaluation value.That is, the average of differences in signal levels corresponding toone variation in the threshold, during past plural symbol timings isobtained, and the obtained average is transmitted to the thresholdevaluation unit 251 as a threshold control signal. The average isobtained considering also that spaces between the thresholds 3 and 4,and the thresholds 4 and 5 are 1.5 times as large as spaces betweenother thresholds in the case of FIG. 11. Then, the threshold evaluationunit 251 changes the threshold in accordance with the threshold controlsignal.

Thus, the threshold is modified on the basis of the evaluation result onthe signals that have been received for a predetermined period.Accordingly, in cases where the transmitted voltage varies according tothe change in the supply voltage or the like, correct data can beobtained by modifying the threshold.

In this embodiment, when modifying the threshold, the evaluation unit250 obtains differences corresponding to one variation in the thresholdto obtain an average. However, any value such as a value correspondingto the maximum amplitude may be obtained so long as this value enablesto modify changes in the amplitude level in the signal waveform whichhas been transmitted from the transmitting end 100, and the thresholdcan be appropriately modified by averaging such values during apredetermined period.

In this embodiment, the modification of the threshold is performed afterthe processing by the digital filter is performed, while the thresholdmay be fixed and the received signal is amplified to change theamplitude into an appropriate level.

Further, in this embodiment, signal levels which are one more than thenumber of symbols are provided, then a signal level of a signal that wastransmitted in the previous symbol timing is employed as a prohibitlevel, and the symbol is mapped to other signal level. However, thenumber of signal levels may be increased, thereby to increase the numberof prohibit levels. For example, increase or decrease of the previoussignal level with relative to a signal level that is antecedent to theprevious signal level is stored and, when the signal level is dropping,signal levels which are lower than the previous signal level areprohibited in the next symbol timing. On the other hand, when the signallevel is rising, signal levels which are higher than the previous signallevel are prohibited. The signal waveform in this case constantly risesand drops repeatedly in each symbol timing, whereby a synchronizationclock having a stable phase is generated on the receiving end.

In this embodiment, the descriptions have been given of the signaltransmission according to the multi-valued baseband transmission, whileemitted noises can be reduced in the same manner as in this embodimentalso in cases where modulation such as ASK (amplitude shift keying) or64QAM (Quadrature Amplitude Modulation) is performed. When themodulation is performed, signal bands are located on both sides of themodulation frequency, so that the frequency required for each symbolrate at the baseband transmission is reduced to half, whereby thelimited band due to the characteristics of the twisted pair cable iseffectively utilized, resulting in higher-speed transmission. Further,when the phase and amplitude are both subjected to the modulation likein 64QAM, more efficient transmission can be performed, therebyrealizing a higher transmission rate also when the same twisted paircable is employed.

INDUSTRIAL AVAILABILITY

The present invention provides a digital data transmission apparatus,and a transmission line coding method and decoding method for convertingdigital data into multi values to be coded so that the same signal leveldoes not successively appear, thereby enabling high-speed datatransmission, as well as reducing noise emission in a band of 30 MHz orlower by means of a digital filter up to the amount that is compliantwith requirements for mounting on motor vehicles.

1. A digital data transmission system comprising: a data coding unitoperable to convert digital data into a signal level corresponding to asymbol that is assigned to the digital data in each symbol cycle as aprescribed unit cycle; a first digital filter having a first samplingcycle that is shorter than a unit cycle of a signal level string whichhas been obtained by said data coding unit, said first digital filterbeing operable to pass only predetermined frequencies; a D/A converteroperable to convert a digital data stream that has passed though saidfirst digital filter into an analog signal; a first low-pass filteroperable to eliminate folding distortion of said first digital filterfrom the analog signal obtained by said D/A converter, which distortionis decided in the first sampling cycle; a differential driver operableto convert an output of said first low-pass filter into two convertedsignals having opposite polarities relative to a predetermined referencepotential, and to differentially output the converted signals; a secondlow-pass filter operable to eliminate a predetermined frequency bandfrom each of the converted signals which are outputted from saiddifferential driver, and to output obtained signals up to a twisted paircable; a differential receiver operable to receive transmission signalswhich are transmitted through the twisted pair cable, and to convert adifference in potential between two wires of the cable pair into asignal; an A/D converter operable to convert the signal outputted fromsaid differential receiver into a digital signal value in each secondsampling cycle; a second digital filter operable to pass only apredetermined frequency band of a digital data stream that has beenobtained by sampling a signal of said A/D converter; and a levelevaluator operable to evaluate a symbol value from a level of a signalin symbol timing, including a symbol in the signal, based on an outputfrom said second digital filter, and to convert the symbol value intocorresponding digital data, wherein said first and second digitalfilters both have low-pass characteristics, and said first digitalfilter has frequency characteristics of cutting off at least frequencydata which are higher than a frequency band in which electromagneticwaves emitted from the respective signals that pass through the twistedpair cable cancel out, thereby to eliminate emission of theelectromagnetic waves to outside the twisted pair cable, and wherein acombination of said first low-pass filter and said second low-passfilter provides low-cut characteristics of eliminating foldingdistortion of said first digital filter, which is decided in the firstsampling cycle.
 2. The digital data transmission system of claim 1,wherein said first and second digital filters have transmissioncharacteristics that exhibit roll-off characteristics in a case wheresignals are passed through said two digital filters.
 3. A digital datatransmission apparatus operable to transmit data signals using a twistedpair cable, said apparatus comprising: a data coding unit operable toconvert digital data into a signal level corresponding to a symbol thatis assigned to the digital data in each symbol cycle as a prescribedunit cycle; a first digital filter having a first sampling cycle that isshorter than a unit cycle of a signal level string which has beenobtained by said data coding unit, said first digital filter beingoperable to pass only predetermined frequencies; a D/A converteroperable to convert a digital data stream that has passed through saidfirst digital filter into an analog signal; a first low-pass filteroperable to eliminate folding distortion of said first digital filterfrom the analog signal obtained by said D/A converter, which distortionis decided in the first sampling cycle; a differential driver operableto convert an output of said first low-pass filter into two convertedsignals having opposite polarities relative to a predetermined referencepotential, and to output the converted signals; and a second low-passfilter operable to eliminate a predetermined frequency band from each ofthe converted signals which are outputted from said differential driver,and to output obtained signals to the twisted pair cable, wherein saidfirst digital filter has low-pass characteristics of cutting off atleast frequency data which are higher than a frequency band in whichelectromagnetic waves emitted from the respective signals that passthough the twisted pair cable cancel out, thereby to eliminate emissionof electromagnetic waves to outside the twisted pair cable, and whereina combination of said first low-pass filter and second low-pass filterprovides low-cut characteristics of eliminating folding distortion ofsaid first digital filter, which is decided in the first sampling cycle.4. The digital data transmission apparatus of claim 3, furthercomprising a common mode choke coil operable to eliminate common modenoises from the signals from which the predetermined frequency band hasbeen eliminated by said second low-pass filter, and to output obtainedsignals to the twisted pair cable.
 5. The digital data transmissionapparatus of claim 3, wherein said first digital filter has transmissioncharacteristics that exhibit roll-off characteristics in the case wheretransmission signals are passed through said first digital filter and asecond digital filter, wherein said second digital filter is included ina digital data receiving apparatus that is connected in a stagesubsequent to said digital data transmission apparatus and comprises adifferential receiver operable to receive transmission signals that aretransmitted from a digital data transmission apparatus having the samestructure as said digital data transmission apparatus and to convert adifference in potential between two wires into a signal; an A/Dconverter operable to convert the signal outputted from the differentialreceiver into a digital signal value in each second sampling cycle; saidsecond digital filter operable to pass only a predetermined frequencyband of a digital data stream that has been obtained by sampling asignal of the A/D converter; and a level evaluator operable to evaluatea symbol value from a level of a signal in symbol timing, including asymbol in the signal, based on an output from said second digitalfilter, and to convert the symbol value into corresponding digital data.6. The digital data transmission apparatus of claim 5, wherein said datacoding unit is further operable to convert data, comprising two or morebits per symbol cycle, into a symbol to be transmitted.
 7. The digitaldata transmission apparatus of claim 6, wherein said data coding unithas signal levels which are more than the number of kinds of symbols tobe transmitted per symbol cycle, and is further operable to assign asymbol in a symbol transmission timing to one of the signal levels. 8.The digital data transmission apparatus of claim 7, wherein said datacoding unit has five signal levels, and is further operable to assign asymbol in a symbol transmission timing to a signal level other than aprevious signal level corresponding to a signal which was transmitted inan immediately preceding symbol transmission timing, in the order of 01,11, 00, 10, starting from a lowest signal level.
 9. The digital datatransmission apparatus of claim 7, wherein digital data to betransmitted have been coded by a bi-phase mark method, and wherein saiddata coding unit is further operable to assign a symbol in a symboltransmission timing to a signal level other than a previous signal levelcorresponding to a signal which was transmitted in an immediatelypreceding symbol transmission timing, in the order of 01, 11, 00, 10,starting from a lowest signal level, thereby to decide a signal level tobe transmitted.
 10. The digital data transmission apparatus of claim 7,wherein said data coding unit includes: a previous level storage unitoperable to store a previous signal level corresponding to the signalwhich was transmitted in an immediately preceding symbol transmissiontiming; and a coding unit operable to decide a signal levelcorresponding to the symbol to be transmitted, based on the previoussignal level and the transmission symbol.
 11. The digital datatransmission apparatus of claim 10, wherein said coding unit is furtheroperable to assign a symbol in a symbol transmission timing to a signallevel having a predetermined difference from the previous signal levelthat is stored in said previous signal level storage unit.
 12. The datatransmission apparatus of claim 11, wherein said data coding unit issupplied with a transmission method instruction signal indicatingwhether the transmission signal has been coded by the bi-phase markmethod.
 13. The data transmission apparatus of claim 7, wherein saiddata coding unit is supplied with a transmission method instructionsignal indicating whether the transmission signal has been coded by thebi-phase mark method.
 14. The digital data transmission apparatus ofclaim 6, wherein said data coding unit has five signal levels, and isfurther operable to assign a symbol in a symbol transmission timing to asignal level other than a previous signal level corresponding to asignal which was transmitted in an immediately preceding symboltransmission timing, in the order of 01, 11, 00, 10, starting from alowest signal level.
 15. The digital data transmission apparatus ofclaim 14, wherein said data coding unit includes: a previous levelstorage unit operable to store a previous signal level corresponding tothe signal which was transmitted in an immediately preceding symboltransmission timing; and a coding unit operable to decide a signal levelcorresponding to the symbol to be transmitted, based on the previoussignal level and the transmission symbol.
 16. The digital datatransmission apparatus of claim 15, wherein said coding unit is furtheroperable to assign a symbol in a symbol transmission timing to a signalhaving a predetermined difference from the previous signal level that isstored in said previous signal level storage unit.
 17. The datatransmission apparatus of claim 16, wherein said data coding unit issupplied with a transmission method instruction signal indicatingwhether the transmission signal has been coded by the bi-phase markmethod.
 18. The data transmission apparatus of claim 14, wherein saiddata coding unit is supplied with a transmission method instructionsignal indicating whether the transmission signal has been coded by thebi-phase mark method.
 19. The digital data transmission apparatus ofclaim 6, wherein digital data to be transmitted have been coded by abi-phase mark method, and wherein said data coding unit is furtheroperable to assign a symbol in a symbol transmission timing to a signallevel other than a previous signal level corresponding to a signal whichwas transmitted in an immediately preceding symbol transmission timing,in the order of 01, 11, 00, 10, starting from a lowest signal level,thereby to decide a signal level to be transmitted.
 20. The digital datatransmission apparatus of claim 19, wherein said data coding unitincludes: a previous level storage unit operable to store a previoussignal level corresponding to the signal which was transmitted in animmediately preceding symbol transmission timing; and a coding unitoperable to decide a signal level corresponding to the symbol to betransmitted, based on the previous signal level and the transmissionsymbol.
 21. The digital data transmission apparatus of claim 20, whereinsaid coding unit is further operable to assign a symbol in a symboltransmission timing to a signal having a predetermined difference fromthe previous signal level that is stored in said previous signal levelstorage unit.
 22. The data transmission apparatus of claim 21, whereinsaid data coding unit is supplied with a transmission method instructionsignal indicating whether the transmission signal has been coded by thebi-phase mark method.
 23. The data transmission apparatus of claim 19,wherein said data coding unit is supplied with a transmission methodinstruction signal indicating whether the transmission signal has beencoded by the bi-phase mark method.
 24. The digital data transmissionapparatus of claim 6, wherein said data coding unit includes: a previouslevel storage unit operable to store a previous signal levelcorresponding to the signal which was transmitted in an immediatelypreceding symbol transmission timing; and a coding unit operable todecide a signal level corresponding to the symbol to be transmitted,based on the previous signal level and the transmission symbol.
 25. Thedigital data transmission apparatus of claim 24, wherein said codingunit is further operable to assign a symbol in a symbol transmissiontiming to a signal having a predetermined difference from the previoussignal level that is stored in said previous signal level storage unit.26. The data transmission apparatus of claim 25, wherein said datacoding unit is supplied with a transmission method instruction signalindicating whether the transmission signal has been coded by thebi-phase mark method.
 27. The data transmission apparatus of claim 24,wherein said data coding unit is supplied with a transmission methodinstruction signal indicating whether the transmission signal has beencoded by the bi-phase mark method.
 28. The data transmission apparatusof claim 6, wherein said data coding unit is supplied with atransmission method instruction signal indicating whether thetransmission signal has been coded by the bi-phase mark method.
 29. Adigital data receiving apparatus operable to receive transmissionsignals that have been transmitted from a digital data transmissionapparatus operable to transmit data signals using a twisted pair cable,the digital data transmission apparatus comprising: a data coding unitoperable to convert digital data into a signal level corresponding to asymbol that is assigned to the digital data in each symbol cycle as aprescribed unit cycle; a first digital filter having a first samplingcycle that is shorter than a unit cycle of a signal level string whichhas been obtained by the data coding unit said first digital filterbeing operable to pass only predetermined frequencies; a D/A converteroperable to convert a digital data stream that has passed through saidfirst digital filter into an analog signal; a first low-pass filteroperable to eliminate folding distortion of said first digital filterfrom the analog signal obtained by the D/A converter, which distortionis decided in the first sampling cycle; a differential driver operableto convert an output of the first low-pass filter into two convertedsignals having opposite polarities relative to a predetermined referencepotential, and to output the converted signals; and a second low-passfilter operable to eliminate a predetermined frequency band from each ofthe converted signals which are outputted from the differential driver,and to output obtained signals to the twisted pair cable, wherein saidfirst digital filter has low-pass characteristics of cutting off atleast frequency data which are higher than a frequency band in whichelectromagnetic waves emitted from the respective signals that passthrough the twisted pair cable cancel out, thereby to eliminate emissionof electromagnetic waves to outside the twisted pair cable, and bycombining the first low-pass filter and the second low-pass filters,wherein a combination of the first and second low-pass filters havelow-cut characteristics of eliminating folding distortion of said firstdigital filter, which is decided in the first sampling cycle, saiddigital data receiving apparatus comprising: a differential receiveroperable to receive transmission signals which are transmitted from saiddigital data transmission apparatus through the twisted pair cable, andto convert a difference in potential between two wires of the cable intoa signal; an A/D converter operable to convert the signal outputted fromsaid differential receiver into a digital signal value in each secondsampling cycle; a second digital filter operable to pass only apredetermined frequency band of a digital data stream that has beenobtained by sampling a signal of said A/D converter; and a levelevaluator operable to evaluate a symbol value from a level of a signalin symbol timing, including a symbol in the signal, based on an outputfrom said second digital filter, and to convert the symbol value intocorresponding digital data.
 30. The digital data receiving apparatus ofclaim 29, wherein said second digital filter has transmissioncharacteristics that exhibit roll-off characteristics in the case wherethe transmission signals are passed through said first digital filterand said second digital filter.
 31. The digital data receiving apparatusof claim 30, wherein said level evaluator is supplied with atransmission method instruction signal indicating whether the receivedsignal has been coded by the bi-phase mark method.
 32. The digital datareceiving apparatus of claim 29, wherein said level evaluator comprisesa signal level detector operable to detect a signal level in each symbolcycle, and a previous signal level storage unit operable to store theprevious signal level which was received in an immediately precedingsymbol receipt timing, and wherein said level evaluator is operable todecode the signal level detected by said signal level detector into acorresponding symbol, based on the previous signal level that is storedin said previous signal level storage unit.
 33. The digital datareceiving apparatus of claim 32, wherein said level evaluator furtherincludes: a threshold controller operable to correct an evaluationthreshold level based on variation values in respective signal levelswhich were received during a predetermined period; a previous signallevel storage unit operable to store a previous signal levelcorresponding to a signal which was received in an immediately precedingsymbol receiving timing; and a threshold evaluator that has a thresholdvalue, and that is operable to perform threshold evaluation to adifference in signal level between a signal level detected in a symbolreceipt timing and the previous signal level, thereby to decode a symbolvalue.
 34. The digital data receiving apparatus of claim 33, whereinsaid level evaluator further includes a synchronization unit operable toestablish synchronization with a symbol cycle of a received signal, toextract frequency components having a half cycle as long as the symbolcycle from the received signal and to control a symbol timing at which asymbol is detected based on a phase of an extracted signal.
 35. Thedigital data receiving apparatus of claim 34, wherein said levelevaluator is supplied with a transmission method instruction signalindicating whether the received signal has been coded by the bi-phasemark method.
 36. The digital data receiving apparatus of claim 33,wherein said level evaluator is supplied with a transmission methodinstruction signal indicating whether the received signal has been codedby the bi-phase mark method.
 37. The digital data receiving apparatus ofclaim 32, wherein said level evaluator is supplied with a transmissionmethod instruction signal indicating whether the received signal hasbeen coded by the bi-phase mark method.
 38. The digital data receivingapparatus of claim 29, wherein said level evaluator includes: athreshold controller operable to correct an evaluation threshold levelbased on variation values in respective signal levels which werereceived during a predetermined period; a previous signal level storageunit operable to store a previous signal level corresponding to a signalwhich was received in an immediately preceding symbol receipt timing;and a threshold evaluator that has a threshold value, and that isoperable to perform threshold evaluation to a difference in signal levelbetween a signal level detected in a symbol receipt timing and theprevious signal level, thereby to decode a symbol value.
 39. The digitaldata receiving apparatus of claim 38, wherein said level evaluatorfurther includes a synchronization unit operable to establishsynchronization with a symbol cycle of a received signal, to extractfrequency components having a half cycle as long as the symbol cyclefrom the received signal and to control a symbol timing at which asymbol is detected based on a phase of an extracted signal.
 40. Thedigital data receiving apparatus of claim 39, wherein said levelevaluator is supplied with a transmission method instruction signalindicating whether the received signal has been coded by the bi-phasemark method.
 41. The digital data receiving apparatus of claim 38,wherein said level evaluator is supplied with a transmission methodinstruction signal indicating whether the received signal has been codedby the bi-phase mark method.
 42. The digital data receiving apparatus ofclaim 29, wherein said level evaluator includes a synchronization unitoperable to establish synchronization with a symbol cycle of a receivedsignal, to extract frequency components having a half cycle as long asthe symbol cycle from the received signal and to control a symbol timingat which a symbol is detected on a phase of an extracted signal.
 43. Thedigital data receiving apparatus of claim 42, wherein said levelevaluator is supplied with a transmission method instruction signalindicating whether the received signal has been coded by the bi-phasemark method.
 44. The digital data receiving apparatus of claim 29,wherein said level evaluator is supplied with a transmission methodinstruction signal indicating whether the received signal has been codedby the bi-phase mark method.